1. Field of the Invention
The present invention relates generally to the field of high fidelity audio reproduction; and, more particularly concerns subwoofer loudspeaker systems that produce high quality, low distortion and low-frequency sound.
2. Prior Art
In the field of high fidelity sound reproduction, a high quality audio system is normally comprised of: a) a signal source, which is generally music or soundtracks from: i) films; ii) compact disk players; iii) laser disk players, and the like; b) a "preamplifier" which receives signals from the signal source and provides an audio signal to a power amplifier which amplifies the signal; and c), loudspeakers that can reproduce the sound from the signal source. Generally, loudspeakers are single enclosures designed to produce most of the audible frequency range, which is from 20 Hertz ("Hz") to 20,000 Hz.
Modern recording technologies have allowed music and film producers to make recordings having wider dynamic ranges--i.e., higher signal-to-noise ratios--and more extended frequency response. Furthermore, many music and film recordings contain more low frequency information than those of only a few years ago. This is especially true in film soundtracks, where recordings of special effects such as explosions are commonplace.
In response to the increased low frequency sound in recordings, a growing number of audio systems are adding an additional type of loudspeaker to their existing array of loudspeakers. This type of loudspeaker is known as a "subwoofer". Subwoofers are specialized loudspeakers which reproduce only the lowest frequencies of the audible frequency range--viz., those frequencies ranging from approximately 20 Hz to about 80 to 120 Hz. These low frequencies are difficult for many full range loudspeakers to reproduce because the bass drivers for full range loudspeakers must handle a wider frequency range--i.e., their frequency response must extend much higher in the audible frequency range, often to about 2,500 Hz or even higher depending upon the design of the loudspeaker. Adding a subwoofer to an audio system relieves the full range loudspeaker from reproducing the lowest frequencies, thereby improving its performance. In addition, certain standards are being set for the reproduction of film soundtracks at home which require the use of one or more subwoofers. Such standards include THX.RTM. (a registered trademark of Lucas Film, Ltd.) certification from Lucas Film and Dolby AC-3 Surround Sound.RTM. (a registered trademark of J. C. Penney Company, Inc.) from Dolby Laboratories. Dolby AC-3 Surround Sound.RTM. even has an audio channel dedicated to only low frequency information.
Conventional design of a subwoofer involves the placement of one or more large bass drivers into a large cabinet--e.g., typically a cabinet enclosing a volume of space ranging from about 8 cubic feet to about 27 cubic feet. Bass drivers, known as "woofers", generally include a circular "diaphragm" or "cone" which can be constructed of many different materials including paper, plastic, kevlar, etc. Woofer cones have a certain diameter--viz., the bore of the cone is equal to pi.times.radius.sup.2 (.pi.r.sup.2). Prior art subwoofer cones capable of high acoustic output generally have a diameter of at least ten inches, and often greater.
The circumference of the cone is affixed to a "surround" or "suspension", which is then affixed to the driver's frame. The suspension enables the cone to move in and out of the driver frame at a particular frequency and returns it to a null position when no sound is produced. The peak-to-peak distance traveled by the cone is known as the "stroke" of the driver--sometimes referred to as the "excursion" of the driver. Generally, the drivers installed in prior art subwoofers have a peak-to-peak stroke or excursion of between 0.4" and 0.6". Prior art suspensions are constructed of flexible, compliant materials such as relatively thin rubber, impregnated cloth, expanded synthetic cellular foam such, for example, as expanded cellular polyethylene ("PE") surround foam, or similar materials which are compressed to a thickness of about 0.02" and which are not self-supporting, which have historically produced very little resistance to peak-to-peak cone movement, and which are capable of standing off box pressures of only on the order of nominally about 0.1 lbs/in.sup.2 and, at best, only about 0.15 lbs/in.sup.2.
Movement of the cone about the suspension causes air to be moved, which is what produces the sound heard and, in the case of bass, felt by the listener. The amount of air that can be moved by a driver is directly related to the bore and stroke of the subwoofer cone. Thus, to increase the amount of air that a subwoofer can move, the bore, the stroke, and/or both the bore and stroke, can be increased. However, and as will be discussed below, simply increasing the bore and/or the stroke has disadvantages.
At the center of the cone, the driver is affixed to the "motor" of the cone which is comprised generally of a single electrical conductor placed within a magnetic field. In the prior art, the electrical conductor is a single electrical wire wrapped around a cylinder. This arrangement is know as the "voice coil" of the driver. The voice coil is wrapped around a voice coil former which is, in turn, affixed to the cone of the driver and placed in proximity to a magnet. When current is run through the voice coil, magnetic fields are created around the voice coil. These voice coil magnetic fields interact with the magnetic fields of the magnet, which causes the voice coil former to move. The voice coil former's movement causes the movement of the cone. Cone movement, as discussed above, causes movement of air which produces sound. Producing sound at higher volumes requires greater cone movements. Greater cone movements are produced when the voice coil and the driver's magnet have greater magnetic field interactions; and, this increased magnetic field interaction is produced when the voice coil has more current running through it.
To reproduce low frequencies at high volume levels, a subwoofer must be capable of moving large quantities of air. Typical prior art subwoofers for use in the home can move approximately one-hundred thirty cubic inches of air. For louder audio volumes, it is desirable that the subwoofer be capable of moving even more air--for example, one-hundred eighty cubic inches of air. A typical fifteen inch diameter woofer, which has a cone diameter of approximately thirteen inches and a stroke of approximately 0.6 inches, can move approximately eighty cubic inches of air. Therefore, generally a prior art subwoofer will utilize two of these drivers; and two drivers are able to move approximately one-hundred sixty cubic inches of air. One disadvantage of having a driver with a fifteen inch cone is that it is difficult to design a cone of that size which is rigid enough to resist distortion when the cone has such a large surface area.
Another example of a prior art subwoofer utilizes four twelve inch drivers. Each of these drivers has a cone diameter of approximately ten inches and a stroke of approximately 0.6 inches. Such a subwoofer can move approximately one-hundred ninety cubic inches of air. However, such a subwoofer suffers from the disadvantage that four drivers are required; and, this greatly increases the size of the cabinet required, cost and weight.
Of course, it is possible to increase the stroke of the driver, and thus increase the amount of air that is moved by the driver. However, when the stroke of the driver is increased, the efficiency of the driver is substantially reduced, as less of the voice coil will remain in the magnetic gap.
Prior art subwoofer systems invariably require a large cabinet. One reason, as seen from the above, is that many prior art subwoofer systems utilize several large drivers so that they can move enough air for adequate performance. However, large cabinets are necessary for prior art subwoofers for reasons having nothing to do with the number of drivers installed therein. Some of the more significant reasons for this are set forth hereinbelow.
Drivers for subwoofers are generally installed in a sealed or vented box. Thus, when the cone of the driver moves, it must overcome the forces inherently created because of the box structure itself. For instance, during operation, if the cone is moving into the cabinet, the air inside the cabinet will be compressed by the moving cone, thereby creating a force resisting inward cone movement. If, on the other hand, the cone is moving out of the cabinet, a vacuum is created that, in effect, exerts a force tending to pull the cone back into the cabinet. These conditions exist for both sealed and vented boxes or cabinets. Atmospheric pressures outside the cabinet also affect these forces.
The driver must overcome the foregoing forces during movement of the cone. The higher the pressure to be overcome (whether positive or negative), the more power that is required to overcome that pressure. The physical structure of the subwoofer can be manipulated to deal with the increase in power that is requited to overcome the foregoing forces. First, a larger enclosure (i.e., cabinet) can be used. A larger enclosure will create less resistance to inward and outward cone movements because it contains more air than a smaller enclosure. The reason for this is that when the driver cone moves into the cabinet, the larger air volume is compressed to a lower pressure. Thus, less power is required by the voice coil to overcome the forces created by the compression of air within the cabinet. Further, when the driver cone moves out of the cabinet, less vacuum is created, which therefore allows the voice coil to move the cone with less power. Because of this, prior art subwoofers have typically utilized relatively large cabinets.
A second design factor is related to the stroke of the driver. If the stroke of the driver is short, the driver cone will have physical limitations on how far it can enter into the cabinet and how far it can extend outwardly from the cabinet. The shorter the extension of the driver cone into the cabinet, the less air that will be compressed within the cabinet. Such a movement will, therefore, require less power into the voice coil to effectuate movement of the cone. The same holds true for cone extension out of the cabinet. The shorter the extension of the driver cone out of the cabinet, the less will be the vacuum that is created and, therefore, the less power that will be required for such cone movement.
Power in prior art subwoofer systems must be provided by power amplifiers. Often a subwoofer system will use a separate power amplifier. However, for ease of packaging, many prior art subwoofer systems utilize power amplifiers that are built into the cabinet of the subwoofer. In general, power amplifiers capable of driving conventional prior art subwoofers must be large and capable of creating between one-hundred (100) to three-hundred (300) watts of power. Large amounts of power are required to drive a subwoofer for many of the reasons described above. However, power amplifiers capable of providing such power levels tend to create large amounts of heat which, in turn, requires large heat sinks, massive power reserve capacitors, and large transformers, all of which are large in size, heavy, and expensive. All of these factors are undesirable; and, all tend to reinforce the need for a relatively large cabinet.
Thus, as can be seen from the foregoing, because of the large power demands required by subwoofer systems and the large cost involved in providing large amounts of power amplification, prior art subwoofer apparatus have invariably required, and utilized, large cabinets which held drivers having large diameters and short strokes. Such an arrangement, as discussed above, allowed the subwoofer to move reasonable amounts of air without distortion. However, normal listening environments often do not have space for such a large cabinet. Therefore, there is a need for a subwoofer system capable of producing low frequency information at high listening volumes that is packaged in a small volume cabinet.
The design of audio woofers has, for many years, been predicated on conventional wisdom commonly referred to as "Hoffman's Iron Law" which provides: EQU Eff.=V.sub.BOX /f.sub.0.sup.3 =kV.sub.BOX /f.sub.0.sup.3 [1]
where f.sub.0 is the desired low frequency cutoff or limit for the subwoofer; V.sub.BOX is the volume of the cabinet; and, Eff. is the efficiency of the subwoofer. Unfortunately, if one wishes to reduce the low frequency cutoff (f.sub.0) from, for example, 50 Hz to 18 Hz while retaining the same efficiency, the volume of the woofer cabinet must be significantly increased. Or, if one wishes to decrease box volume from, for example, 1 ft.sup.3 to 0.4 ft.sup.3 and, at the same time, decrease the low frequency cutoff (f.sub.0) from, for example, 50 Hz to 18 Hz, efficiency drops by a factor of approximately 53. Consequently, the woofer designer finds that where a 50 watt or 100 watt amplifier might have operated a 1 ft.sup.3 woofer at a 50 Hz low frequency cutoff, a 0.4 ft.sup.3 box at 18 Hz low frequency cutoff will require an amplifier that is approximately 53 times larger than conventional.
For example, a typical loudspeaker in a 1 ft.sup.3 box with a low frequency cutoff of 50 Hz and one percent (1%) efficiency will normally operate satisfactorily if it employs a 200 watt amplifier. But, were the designer to arbitrarily decide to reduce the box volume to 0.4 ft.sup.3 and the low frequency cutoff to 18 Hz, the wattage requirement for the amplifier would be 10,600 watts. That, of course, would be ludicrous and is neither practical, cost effective nor economically feasible from a commercial standpoint.
In essence, Hoffman's Iron Law forbids one from making a subwoofer having a small volume box, high efficiency and low frequency cutoff; and, designers of subwoofers have not deviated from religious adherence to such theories. If the speaker designer wants to have a highly efficient bass driver for a highly efficient woofer that can have a very low frequency cutoff, the box must be huge--and, they always are. Conversely, if the designer wishes the box to be small, there has heretofore been no way to get a lot of bass out of it, either low or loud, with high efficiency.
At the same time, speaker designers have been taught, and have believed, that there is an optimum size for magnets employed in voice coil driven woofers--i.e., it has been assumed that if the magnet is too small, the speaker will not work at all; but, if the magnet is too large, only a small percentage of the output wattage from the power amplifier will be applied to the voice coil. Consequently, woofer designers have concluded that an optimum magnet must lie somewhere between "too small" and "too large" in order to produce effective power in the voice coil. Typically, therefore, virtually all conventional subwoofers will employ a magnet that weighs on the order of only about 20 ounces or less. Indeed, even in the face of today's highly advanced technologies, speaker designers still believe that a well designed, commercially marketable subwoofer should employ: i) a relatively large cabinet--e.g., from about eight to about twenty-seven ft.sup.3 ; ii) multiple large drivers; iii) drivers with peak-to-peak strokes generally on the order of not more than 0.4" to 0.6"; iv) magnets weighing, on average, not more than 20 ounces and, at the very most, about 40 ounces; v) low internal box pressures of on the order of only about 0.1 lbs/in.sup.2 ; and, vi), surrounds or suspension systems that are very compliant leading to surrounds that are, at best, flimsy and incapable of stably supporting the moving driver components without wobble and consequent degradation of the audio sounds generated.
The problem of attempting to design a woofer which is small in size--e.g., defining an enclosed volume of space of about 0.4 ft.sup.3 to about 0.5 ft.sup.3 having a low cutoff frequency below about 40 Hz, and which is, at the same time, efficient, has defied solution--at least until the advent of the present invention and the invention disclosed in Applicant's aforesaid co-pending U.S. patent application, Ser. No. 08/582,149, filed Jan. 2, 1996, now U.S. Pat. No. 5,748,753, issued May 5, 1998. For example, as stated by Louis D. Fielder of Dolby Laboratories, Inc. and Eric M. Benjamin in an article entitled "Subwoofer Performance for Accurate Reproduction of Music", J. Audio Eng. Soc., Vol. 36, No. 6, June 1988, pages 443 through 454 at page 446:
"For the required value of 0.0316 acoustic W at 20 Hz, this results in a volume excursion of 41.8 in.sup.3 (685 cm.sup.3). For a single 12-in (0.3-m) woofer [effective piston diameter 10 in (0.25 m)] this would require a peak linear excursion of 0.53 in (13.5 mm). This large excursion requirement can be reduced by using larger drivers, increasing the number of drivers, and utilizing the low-frequency boost provided by the room. With four 15-in (0.38-m) woofers the peak linear excursion required is 0.078 in (2 mm), neglecting room effects."
In short, the "solution" advocated by the authors, who are accredited experts that were then attempting to establish design criteria for the performance of subwoofers to be used for the reproduction of music in the home, is: i) to design a woofer having a peak linear excursion of 0.53"; ii) to attempt to reduce this "large excursion"--i.e., 0.53"--by using larger drivers and increasing the number of drivers (and, therefore, the size of the box or subwoofer cabinet); and iii), utilizing the low frequency boost provided by the listening room.
Those skilled in the art relating to subwoofers will recognize that the efficiency of a subwoofer is proportional to the size of the box or cabinet that the subwoofer is mounted in. Therefore, a box or cabinet that is 1/10th the size of a conventional prior art subwoofer box or cabinet would ordinarily be ten times less efficient than its prior art counterpart. Under those circumstances, ten times more heat is developed in the voice coil regardless of the efficiency of the driving amplifier. Consequently, the voice coil will soon overheat; and, in fact, that has been a major stumbling block to the development of very small, but powerful, subwoofers. Nevertheless, as will become apparent from the ensuing description, the present invention relates specifically to a subwoofer characterized by its high efficiency and, at the same time, its extremely small box or cabinet.
The broad concept of the present invention, in fact, flies in the face of all known subwoofer computer modeling programs as well as the teachings in the prior art literature.
In this connection, those skilled in the art will appreciate that raw driver efficiency is expressed as: EQU Eff.=(Bl).sup.2 /r.sub.e [2]
where "B" is the magnetic field strength, and "l " and "r.sub.e " are constants.
Rewriting equation [2] it is found: EQU Eff.=kB.sup.2 [3]
Based upon the foregoing, those skilled in the art will understand that in a subwoofer driver where B is increased by a factor of 3.3, the efficiency will be increased by a factor of approximately 10--viz., 3.3.sup.2 .apprxeq.10. Unfortunately, however, when such a subwoofer driver is built and installed in a box--any box--bass output is found to be actually far less than before the magnetic field was increased! This fact is well known to those skilled in the subwoofer art; and, consequently, prior art conventional subwoofers have evolved with magnetic fields optimized for maximum bass output.
Unfortunately, subwoofers designed with magnets optimized for maximum bass output are very inefficient. The reason for this is because the motor of the subwoofer (consisting of the voice coil and magnetic structure) is operating very close to stall, a condition characterized by relatively high armature winding--or, in the case of subwoofers, voice coil--heating. By increasing the magnetic field strength, the efficiency is increased, but the bass output is decreased because of the large back emf generated by the motion of the subwoofer's voice coil immersed in its magnetic field. The magnitude of the back emf is established by Lenze's Law: EQU back emf=d.0./dt, [4]
where .0. is the magnetic flux.
The back emf generated acts to prevent current from flowing in the voice coil because it opposes the forward voltage impressed on the voice coil winding. With the lowered current in the voice coil, the result is less bass.
It must be recognized at this point that all prior art literature known to the inventor, the descriptive equations therein, and all subwoofer computer modeling programs based on prior art literature make the basic assumption that the subwoofer is operating in stall in order to simplify the modeling. Prior to the advent of the present invention, this assumption was tenable because a tracking downconvertor drive amplifier able to deliver the high voltage necessary to overcome the back emf did not exist. Indeed, prior art subwoofer designers have all made the simplifying assumption that the back emf at system impedance minimums is not significant.
Another major problem encountered by subwoofer designers is directly related to the fact that subwoofers are exceptionally prone to hum problems induced by power line "ground loops". Ground loops are caused by a redundant ground that runs from the wall plug or other suitable A.C. source where the subwoofer is plugged in, through the power line to where the audio signal source--e.g., a CD player, an F.M. tuner, a turntable, etc.--is plugged into the power line, and then back to the subwoofer audio input through the audio cable shields. This constitutes a loop called a "ground loop"; and, it generates a very undesirable 60 Hz hum.
Prior art subwoofers all suffer from unwanted "ground loop" induced 60 Hz hum to a greater or lesser degree. Subwoofer designers have attempted to solve the "ground loop" induced 60 Hz hum problem in various ways. One proposed solution includes the use of a balanced transformer which breaks the loop by virtue of its primary and secondary windings. The transformer can either be at the power line input (power transformer), or at the audio input (input transformer), or, for that matter, at both locations. Another attempted solution involves the use of optical couplings in which the audio signal is coupled by a light beam--i.e., there is no ground connection. Both of the foregoing approaches have been effective in substantially reducing, but not eliminating, "ground loop" induced 60 Hz hum problems. This is because while they effectively "break" the ground(s), they do not suppress the hum voltage generated across the broken ground or grounds.